For me, there are three glaring improbabilities from Shakespeare's "The Merchant of Venice." I don't have a particular order or preference to them either.
First, I find it really hard to believe that Portia was able to cross dress so effectively that nobody in the courtroom, not even her husband, recognized her. Sure there are plenty of real historical women that cross dressed and pulled off being a man, but I just don't see it being possible with Portia. She's supposed to be gorgeous and not look anything like a man to begin with.
Second, I find it ridiculous that a court system would even begin to contemplate legally allowing a plaintiff to be paid with a literal pound of flesh.
Third, I find it improbable that so many potential suitors (for Portia) all failed at choosing the correct casket. There's only three choices. Surely word would have gotten around as to which casket persons "A," "B," "C," etc. chose. It wouldn't take long for somebody to take a 1 in 3 chance on the casket that's never been picked before.